Anesthesiologists use
mechanical ventilators in the operating room, on patients who are
paralyzed to facilitate surgery. Most of these are “bag in bottle”
mechanical bellows which are controlled by three factors: 1) tidal volume,
2) respiratory rate, 3) I:E ratio. The I:E is the ratio of time spent in
inspiration versus expiration. Inspiration is active. Expiration is
passive and thus requires more time to allow alveolar units to empty. If
expiration is of insufficient duration, gas is trapped in the alveoli (at
end expiration), a process known as “auto-PEEP”
(click here). If the patient is to receive ten breaths per minute, then the
duration of the cycle is 6 seconds. A conventional I:E is 1:2, so 2
seconds are set aside for inspiration and 4 for expiration. PEEP is rarely
used in the OR, and the combination of reduced FRC (due to tracheal
intubation) and monotonous ventilation leads to a considerable amount of
atelectasis in many patients.

Conventional anesthesia ventilator: the
patient is delivered mandatory breaths from a “bag in bottle” ventilator.
He can also draw unsupported spontaneous breaths from an in-line reservoir
bag: intermittent mandatory ventilation is constructed from this
mechanism.

The main advantage of volume
controlled ventilators is guaranteed minute ventilation. This is
particularly important in the operating room, where lung compliance may be
influenced by the type of surgery involved (abdominal or chest surgery),
and in the ICU or in transit if patient’s tidal volumes are not being
continuously monitored.

Early intensive care
ventilators represented a continuation of operating room techniques, where
the patient was heavily sedated and paralyzed until the disease process
resolved. The problem, though, was how to get the patient off the
ventilator before their muscles atrophied. This required some form of
patient-ventilator interaction.

There is a considerable
difference between mandatory and spontaneous breaths. In mandatory
ventilation the patient is a passive object receiving gas as determined by
the ventilator at a set rate and volume (or pressure). A spontaneously
breathing (awake) individual demands gas at a flow and rate of their own
choosing. Assisted ventilation thus requires a triggering device and a
flow of gas to match the patient’s peak inspiratory demand (30 to 60
liters per minute). The two methods developed to overcome these problems
were assist-control ventilation and intermittent mandatory ventilation.

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